PURPOSE: To perform a steady tracking irrespective of a size and a distance of a target and a change in a focus distance of an optical system, by directly finding a center of gravity of brightness of a target image.

CONSTITUTION: An image of a target irradiated with laser beam obtains an output 30 formed on a first detector 28 by an optical system 4 and amplified by an amplifier 29. Simultaneously, it is divided into two parts by semitransparent mirrors 31 and 32, while the one enters an elevating direction error detecting system 33 and the other enters a revolving direction error detecting system 34. Incident beam permeates a damper 36 having transmissivity changing in an elevating direction, a target image is formed on a second detector 36 equivalent to the detector 28, and an output 37 of the detector 36 amplified by an amplifier 38 is obtained. In which case, the center of gravity in an elevating direction of a target image is represented by a formula of (output 27b× output 30)/a × output 30, and is decided in a uniform manner by the outputs 30 and 37. The center of gravity signal is inputted as an error signal to a controller. Further, a revolving direction is processed in a similar manner.